This invention relates in general to apparatus for drying products and more particularly to a radiant heater for use in a microwave chamber and a microwave drying apparatus which utilizes such a heater.
Certain food products and seeds respond quite well to microwave radiation and are dried most effectively with such radiation. Generally speaking, any food product or seed which tends to form a hard shell when dried using more conventional heating procedures, such as hot air or infrared radiation, is a good candidate for microwave drying, for the microwave radiation heats the product generally from the inside out. Moisture evaporates more quickly from any product when the pressure of the environment in which that product is located is least, and accordingly some microwave dryers for food products subject the products to the microwave radiation in a vacuum chamber. U.S. Pat. Nos. 4,015,341 and 4,347,670 disclose vacuum-type microwave dryers.
Some products are very sensitive to temperature. If the temperature is too high in any part of the product, desirable characteristics of that product can deteriorate rapidly. To produce an optimal product at economic drying rates it is desirable to have the product temperature uniform and at the maximum value that the product will sustain without degradation. Microwave heating, by itself, tends to produce a rather uniform heating, at least when the product thicknesses is less than about a quarter wavelength of the microwave energy. However, if the product is in an environment where the surrounding atmosphere and walls are at significantly lower temperature than the product, the product tends to lose heat from its surface, resulting in a nonuniform temperature in the product. To make drying successful with microwaves alone it is necessary to dry it at a rate such that the interior never reaches the critical temperature at which the product deteriorates. This requirement may make the process time uneconomical.
On the other hand, infrared radiation, which may be derived from a simple heated panel, has just the opposite effect, that is to say, it heats from the outside in, so generally the exterior of any product heated with that type of radiation is hotter than the interior. Again, the product temperature is nonuniform and the drying rate is restricted such that the outer surface does not exceed the critical temperature while the inside dries. This requirement may extend the processing time so much that the process is uneconomical.
Attempts have been made to combine microwave and infrared heating devices so that the interior heating characteristics of the former offset the exterior heating characteristics of the latter and thus provide a more uniform temperature gradient in the product being dried, but these attempts have met with limited success. The primary problem resides in rendering the devices for delivering the two types of radiation to the product at the same location compatable so one does not adversely affect the other. In this regard, microwave energy is usually directed into a vacuum chamber and allowed to reflect about the chamber until it is absorbed by the product. Infrared energy, on the other hand, is normally emitted from a panel or some other surface which must be quite large and relatively close to the product to effect an adequate transmission of heat. Moreover, to effectively radiate, it should be formed from metal, but metal is opaque to microwave energy. Thus, when microwave and infrared heating devices are employed simultaneously at the same location, the metal microwave panel blocks much of the microwave energy.
In the case of a product which is transported on a belt through a vacuum chamber into which microwave radiation is introduced, the microwave energy will reach the product only a very limited distance inwardly from the sides of the belt. This restricts the width of the belt, requiring that it be considerably narrower than is desirable for efficient operation.
U.S. Pat. No. 4,640,020 discloses a microwave dryer in which the microwave drying is achieved in a vacuum chamber that also contains infrared panels, but the microwave and infrared drying do not occur simultaneously or at the same location, and thus the product being dried does not experience a uniform temperature gradient.